Signal attenuation measurements generally involve transmitting a signal towards or through a medium under analysis, detecting the signal transmitted through or reflected by the medium and computing a parameter value for the medium based on attenuation of the signal by the medium. In simultaneous signal attenuation measurement systems, multiple signals are simultaneously transmitted (i.e., two or more signals are transmitted during at least one measurement interval) to the medium and detected in order to obtain information regarding the medium.
Such attenuation measurement systems are used in various applications in various industries. For example, in the medical or health care field, optical (i.e., visible spectrum or other wavelength) signals are utilized to monitor the composition of respiratory and anesthetic gases, and to analyze tissue or a blood sample with regard to oxygen saturation, analyte values (e.g., related to certain hemoglobins) or other composition related values.
The case of pulse oximetry is illustrative. Pulse oximeters determine an oxygen saturation level of a patient's blood, or related analyte values, based on transmission/absorption characteristics of light transmitted through or reflected from the patient's tissue. In particular, pulse oximeters generally include a probe for attaching to a patient's tissue site such as a finger, earlobe or nasal septum. The probe is used to transmit pulsed optical signals of at least two wavelengths, typically red and infrared, to the patient's tissue site. The different wavelengths of light used are often referred to as the channels of the pulse oximeter (e.g., the red and infrared channels). The optical signals are attenuated by the patient tissue site and subsequently are received by a detector that provides an analog electrical output signal representative of the received optical signals. The attenuated optical signals as received by the detector are often referred to as the transmitted signals. By processing the electrical signal and analyzing transmitted signal values for each of the channels at different portions of a patient pulse cycle, information can be obtained regarding blood oxygen saturation or blood analyte values.
Such pulse oximeters generally include multiple sources (emitters) and one or more detectors. A modulation mechanism is generally used to allow the contribution of each source to the detector output to be determined. Some pulse oximeters employ time division multiplexing (TDM) signals. As noted above, the processing of the electrical signals involves separate consideration of the portions of the signal attributable to each of the sources. Such processing generally also involves consideration of a dark current present when neither source is in an “on” state. In TDM oximeters, the sources are pulsed at different times separated by dark periods. Because the first source “on” period, the second source “on” period and dark periods occur at separate times, the associated signal portions can be distinguished for processing.
Alternatively, some pulse oximeters may employ frequency division multiplexing (FDM) signals. In the case of FDM, each of the sources is pulsed at a different frequency resulting in detector signals that have multiple periodic components. Conventional signal processing components and techniques can be utilized to extract information about the different frequency components.
Another type of multiplexing that may be employed in pulse oximeters is code division multiplexing (CDM). In the case of CDM, each of the sources is pulsed in accordance with a unique code comprising a series of high (e.g., +1) and low (e.g., 0) values that identifies each channel from the other channel(s). The codes that are used generally have a number of preferred characteristics, including a random or pseudo-random pattern, orthogonality or near-orthogonality with respect to one another, a substantially equal number of high and low values within a particular code sequence, a relatively even distribution of high and low values within a particular code sequence, and a substantially even distribution of transitions between high and low values within a particular code sequence.
In order to accurately determine information regarding the subject, it is desirable to minimize noise in the detector signal. Such noise may arise from a variety of sources. For example, one source of noise relates to ambient light incident on the detector. Another source of noise is electronic noise generated by various oximeter components, as well as other electrical and electronic equipment within the vicinity of the pulse oximeter. Many significant sources of noise have a periodic component.
Various attempts to minimize the effects of such noise have been implemented in hardware or software. For example, various filtering techniques have been employed to filter from the detector signal frequency or wavelength components that are not of interest. However, because of the periodic nature of many sources of noise and the broad spectral effects of associated harmonics, the effectiveness of such filtering techniques is limited. In this regard, it is noted that both TDM signals and FDM signals are periodic in nature. Accordingly, it may be difficult for a filter to discriminate between signal components and noise components having a similar period. Furthermore, source signal separation can suffer when using CDM signals if there are post-calibration changes in the dynamics of the system. Such post-calibration changes are common given the typically different response rates of different detectors.